In recent years, there has been increased frequency in use of copying machines and printers employing an electrophotographic method accompanied with the development of electronic devices. An active research has been made in the field of an electrophotographic photoreceptor used for an image-forming apparatus employing an electrophotographic method to obtain a highly sensitive electrophotographic photoreceptor (hereafter, it may be simply called as “a photoreceptor”) and a charge generating material used for the photoreceptor.
For example, Patent document 1 (JP-A No. 2012-198278) discloses a highly sensitive photoreceptor in which a 2,3-butanediol adduct of titanyl phthalocyanine is contained as a charge generating material in a charge generating layer. This compound is highly sensitive and hardly deteriorating the performance by the environmental change. Further, titanium oxide particles excellent in electron transporting property are contained in an intermediate layer in the photoreceptor.
In the high sensitive photoreceptor as described above, there will often remain carriers produced in an exposed portion in a first rotation. As a result, it is difficult to obtain a uniform surface electric potential during a charging process in a second rotation. Consequently, it may be produced a so-called exposure memory in which the exposed portion in the first rotation is distinctly observed in the half-tone image produced in the second rotation.
Here, in order to restrain the generation of the exposure memory, it is efficient to incorporate metal oxide particles having a low electron transporting property in the intermediate layer, or to incorporate a charge transporting material having a large ionization potential in the charge transporting layer. However, this will produce a problem that the image density will be changed due to the decrease of the sensitivity after the repeated usage.
Therefore, it is required a highly sensitive photoreceptor that hardly produces an exposure memory, and produces an invariable image density even after repeated usage.